Soybean, Glycine max (L.) Merril, is a major economic crop worldwide and is a primary source of vegetable oil and protein (Sinclair and Backman, Compendium of Soybean Diseases, 3rd Ed. APS Press, St. Paul, Minn., p. 106. (1989). The growing demand for low cholesterol and high fiber diets has also increased soybean's importance as a health food.
Soybean varieties grown in the United States have a narrow genetic base. Six introductions, ‘Mandarin,’ Manchu, ‘Mandarin’ (Ottawa), “Richland,’ ‘AK’ (Harrow), and ‘Mukden,’ contributed nearly 70% of the germplasm represented in 136 cultivar releases. To date, modern day cultivars can be traced back from these six soybean strains from China. In a study conducted by Cox et al., Crop Sci. 25:529-532 (1988), the soybean germplasm is comprised of 90% adapted materials, 9% unadapted, and only 1% from exotic species. The genetic base of cultivated soybean could be widened through exotic species. In addition, exotic species may possess such key traits as disease, stress, and insect resistance.
Soybean aphid, Aphis glycines Matsumura, was identified as new insect pest of soybeans in 2001 and spread to over 21 states in the United States and 3 Canadian provinces by 2003 (Vennette et al. Ann Entomol Soc Am 97:217-226 (2004)). High yields are critical to a farmer's profit margin. Soybean aphid can cause over 50% yield losses (Wang et al., Plant Protect 20:12-13 (1994)). In addition to the decrease in yield, an increase in insecticide use can also decrease a farmer's profit margin. Over 7 million acres of soybean in the North Central U.S. were sprayed with insecticide to control soybean aphids in 2003; the estimated cost of the insecticide treatments was $84-$105 million in the North Central region alone in 2003 (Landis et al. NCR-125 Arthropod biological control: state reports for 2003; Li et al., Mol Breeding 19:25-34 (2007)).
Soybean aphids can directly damage the plant by removing significant amounts of water and nutrients causing the leaves to yellow and wilt. Additionally, aphids excrete honeydew, a sugar-rich sticky substance, on to the leaves and plants. Honeydew often leads to the development of sooty mold, which affects photosynthesis resulting significant yield losses (Gomez et al., Environ Exp Bot 55: 77-86 (2006)). Soybean aphids vector a number of viruses that can stunt plant growth, distorts leaves, cause mottling of leaves and stem, reduce pod number and cause discoloration in the seed. Viruses transmitted via soybean aphid include, Soybean mosaic virus, yellow mosaic virus, tobacco etch virus and tobacco vein mottling virus (Wang et al. Plant Dis 90: 920-926 (2006)).
Host plant resistance to insect are often quantitatively inherited traits and not major resistance gene. Stacking quantitative resistances is more durable than a major gene for resistance, but is difficult to identify and incorporate multiple quantitative resistances into a single soybean variety. Molecular markers associated with insect resistance offers breeders a more efficient method to work with quantitative traits and insect resistance. Aphid resistance genes and QTLs in soybean are known. Examples of which including Rag1 was identified in the soybean variety Dowling and mapped to linkage group M (U.S. patent application Ser. No. 11/158,307). Additionally, quantitative trait loci associated with aphid resistance were identified in Plant Introduction (PI) 567598B and mapped linkage groups B2, D1b, J and K (PCT/US2006/019200).
There is a need in the art of plant breeding to identify additional quantitative trait loci associated with aphid resistance in soybean. Additionally, there is a need for rapid, cost-efficient method to assay the absence or presence of aphid resistance loci in soybean. The present invention provides a method for screening and selecting a soybean plant comprising a quantitative trait loci associated with aphid resistance using single nucleotide polymorphism (SNP) technology.